BAGHDAD, Iraq (CNN) -- The remains of two U.S. contractors who were kidnapped in Iraq have been found, FBI officials said Monday.

The bureau identified the two as Ronald Withrow of Roaring Springs, Texas, abducted on January 5, 2007, and John Roy Young of Kansas City, Missouri, who was captured on November 16, 2006.

Withrow worked for Las Vegas, Nevada-based JPI Worldwide Inc., and Young worked for Crescent Security Group.

The FBI said it had notified the families of the contractors.

Meanwhile, four U.S. soldiers died Sunday night in a roadside bombing in Iraq, military officials reported, bringing the American toll in the 5-year-old war to 4,000 deaths.

The four were killed when a homemade bomb hit their vehicle as they patrolled in a southern Baghdad neighborhood, the U.S. military headquarters in Iraq said. A fifth soldier was wounded.

The grim milestone comes less than a week after the fifth anniversary of the start of the war.

"No casualty is more or less significant than another; each soldier, Marine, airman and sailor is equally precious and their loss equally tragic," said Rear Adm. Gregory Smith, the U.S. military's chief spokesman in Iraq.

Don't Miss

"Every single loss of a soldier, sailor, airman or Marine is keenly felt by military commanders, families and friends both in theater and at home."

Of the 4,000 U.S. military personnel killed in the war, 3,263 have died in attacks and fighting and 737 in nonhostile incidents, such as traffic accidents and suicides. Eight of those killed were civilians working for the Pentagon. The numbers are based on Pentagon data counted by CNN. Check out a company that makes headstones for fallen U.S. troops »

President Bush made remarks about lives lost in Iraq at the State Department on Monday.

"One day, people will look back at this moment in history and say, 'Thank God there were courageous people willing to serve, because they laid the foundations for peace for generations to come,' " he said. "I have vowed in the past and I will vow so long as I'm president to make sure that those lives were not lost in vain; that, in fact, there's an outcome that will merit the sacrifice that civilian and military alike have made."

Also Sunday, at least 35 Iraqis died as the result of suicide bombings, mortar fire and the work of gunmen in cars who opened fire on a crowded outdoor market. Nearly 100 were wounded in the violence.

Another 2 million Iraqis have been forced to leave the country, and 2.5 million have been displaced from their homes within Iraq, according to the U.N. High Commissioner for Refugees.

Many of the Iraqis and U.S. troops killed over the years, like the four soldiers slain Sunday in Baghdad, have been targeted by improvised explosive devices -- the roadside bombs that have come to symbolize Iraq's tenacious insurgency. Watch how the bombs have become a deadly staple »

The Pentagon's Joint Improvised Explosive Device Defeat Organization has been developed to counter the threat of roadside bombs in Iraq as well as Afghanistan. The group calls such bombs the "weapon of choice for adaptive and resilient networks of insurgents and terrorists."

Nearly 160,000 U.S. troops remain in Iraq, and the war has cost U.S. taxpayers about $600 billion, according to the House Budget Committee.

Senior U.S. military officials are preparing to recommend to Bush a four- to six-week pause in additional troop withdrawals from Iraq after the last of the so-called surge brigades leaves in July, CNN learned last week from U.S. military officials familiar with the recommendations but not authorized to talk about them.

The return of all five brigades added to the Iraq contingent last year could reduce troop levels by up to 30,000 but still leave about 130,000 or more troops in Iraq.

Also Monday, the U.S. military said six people killed in a weekend attack were "terrorists" and not members of an American-backed militia, as initially reported.

Those first reports suggested the area of Saturday's helicopter strike may have been a Sons of Iraq checkpoint. Such groups are generically referred to as Awakening Councils -- largely Sunni security forces that the U.S. military have recruited.

A police official in the north-central city of Samarra said the helicopter mistakenly hit a Sons of Iraq checkpoint, killing the six. But the U.S. military said that it believes those killed were not part of the Sons of Iraq.

"I can tell you that two of these individuals were fiddling with something on the side of the road and trying to hide themselves under a blanket when they heard the helicopter," said Maj. Bradford Leighton. "The location of the checkpoint was not at or near any known Sons of Iraq checkpoint."

A joint Iraqi-U.S.-led coalition force is investigating the deaths.

Other developments

Two U.S. government employees -- an American and a Jordanian -- were seriously wounded Sunday when projectiles were fired into Baghdad's International Zone, according to a U.S. government official. He said Monday that the two are in stable condition. Mortars and possibly rockets were fired early Sunday in the area, also known as the Green Zone, site of Iraqi government offices and the U.S. and British embassies. Watch the Green Zone under fire »

Three insurgents were killed and eight were detained during Sunday and Monday raids targeting al Qaeda in Iraq, the U.S. military said. Troops on Monday killed an "armed terrorist" in Tikrit and another in western Baghdad, the military said. Two people were detained in Tikrit and six in Baghdad. An armed man was killed in a raid Sunday in the Hamrin Mountains.